This invention relates to a low level current sensing device and more particularly to a ground fault interrupter circuit for sensing an undesired ground current fault or other hazardous condition in the current supply conductors of an electrical distribution system and for automatically interrupting the current flow to a utility device upon the detection of the ground fault.
Ground Fault Circuit Interruption (GFCI) devices currently available utilize a differential current transformer inductively coupled with the line conductors of a distribution system providing electrical power to a load. The transformer is also provided with a multi-turn secondary winding for sensing a current imbalance in the aforesaid line conductors and in response thereto it develops an output signal proportional to the current imbalance to initiate an interruption of the current supplied to the load via the line conductors. Under normal conditions, i.e. in the absence of a fault on the line conductors, all of the current flowing to the load through one conductor returns to the power source through the other conductor so that the respective currents in the two line conductors are equal in magnitude and introduce equal and oppositely directed flux fields in the transformer core, resulting in a zero secondary output voltage. However, if a line conductor experiences a ground falult condition, such as occurs when a human body comes in contact therewith, a portion of the current returns to the power source via an external ground circuit rather than the neutral line conductor. As a result of the ground fault, the currents in the line conductors, i.e. in the primary windings of the differential transformer, are no longer equal so that a resultant flux is established in the transformer core. If the ground fault current exceed a predetermined threshold level, e.g. 5 milliamps, the GFCI device is designed so that the resultant core flux induces a signal voltage in the transformer secondary winding of a magnitude to initiate, after amplification, operation of a circuit breaker which opens the line conductors to the load.
Another problem that exists in many of the prior art differential transformer ground fault protective circuits relates to the reduced sensitivity of a GFCI device to a ground fault in the event of an inadvertent grounding of the neutral line conductor on the load side of the distribution system. In a power distribution system where the neutral conductor is grounded at the power source side, an inadvertent ground occurring on the neutral conductor at the load side may render the protective circuit ineffective. This problem occurs because a ground fault on the neutral conductor results in the neutral conductor becoming a shorted or low impedance winding of the differential transformer. In order to overcome this problem, a grounded neutral conductor fault condition must be detected by the circuit, or the circuit must be made insensitive to the aforesaid ground fault condition. Several systems have been developed to overcome this problem. For example, U.S. Pat. No. 3,473,091 discloses a ground leakage differential protective circuit in which an impedance is inserted in the neutral line to detect a low impedance to ground in the neutral conductor. U.S. Pat. No. 3,611,035 describes the use of a high frequency tickler coil to induce a high frequency voltage on the neutral conductor for detecting a low impedance to ground in the neutral conductor.
Other forms of grounded neutral protection circuits have been proposed in ground fault circuit interrupters which are generally characterized by the employment of an auxiliary transformer core inductively coupled to the neutral conductor. The present invention however pertains to GFCI devices generally and does not require for its practice a particular form of grounded neutral protection circuit from among those known in the art.
Although prior art ground fault interrupters utilizing the differential transformer technique are useful, they nevertheless are subject to certain inherent limitations which it is an object of the present invention to overcome. For example, in order for a prior art differential transformer GFCI device to sense an unbalanced current flow of the order of 5 milliamps or less in the supply line conductors, it is usually necessary to provide a secondary sensing winding containing a thousand or more turns. Despite the large number of turns on the sensing winding, a low level signal was nevertheless induced therein which generally required additional means, such as an electronic amplifier, to raise the signal to a level sufficient to operate a line conductor circuit breaker or the like. The requirement of an amplifier to amplify the signal to an adequate level to trip a circuit breaker increases the size and cost of the GFCI device. In addition, the amplifier itself is a source of "noise" which is amplified along with the desired output signal. This reduces the overall signal-to-noise ratio of the ground fault interrupter device. Thus, the sensitivity of the ground fault detector is limited to fault signals substantially in excess of the maximum noise levels produced by the amplifier.
In addition, because the output signal of the differential transformer is dependent upon the rate of change in magnetic flux ##EQU1## the output sensitivity of the differential transformer is greatly reduced at low frequencies and DC signals cannot be detected at all. At the customary 60 HZ line frequency in use today, transformer design theory generally requires the use of high permeability core materials, a large number of winding turns and cores having large cross-sectional areas and short magnetic path lengths. These limitations all contribute to the high costs associated with the differential transformer GFCI devices.